Mevalonate (C.sub.6 H.sub.11 O.sub.4) is the metabolic precursor of a vast array of compounds vital for cell and organism viability. In plants, the major endproducts derived from mevalonate are the sterols and other isoprenoids. (see FIG. 1).
Exemplary plant isoprenoids include the terpenes (volatile C.sub.10 and C.sub.15 compounds giving rise to fragrances of many plants) the carotenoids (C.sub.40 compounds giving rise to the color of many plants) and polymers such as natural rubber.
Free sterols are constituents of virtually all eukaryotic membranes. The most abundant sterols of vascular plants are campesterol, 24-methylcholesterol, sitosterol and stigmasterol.
Mevalonate is formed from the reduction of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA). The reduction of HMG-CoA to mevalonate is catalyzed by the enzyme HMG-CoA reductase.
The HMG-CoA reductase enzymes of animals and yeasts are integral membrane glycoproteins of the endoplasmic reticulum. The intact enzyme comprises three regions: a catalytic region, containing the active site of the enzyme, a membrane binding region, anchoring the enzyme to the endoplasmic reticulum and a linker region, joining the catalytic and membrane binding regions of the enzymes. The membrane binding region occupies the NH.sub.2 -terminal portion of the intact protein, whereas the catalytic region occupies the COOH-terminal portion of the protein, with the linker region constituting the remaining portion. Basson, M. E. et al., Mol. Cell Biol., 8(9):3797-3808 (1988). At present, the sub-cellular localization of HMG-CoA reductase in plants is not known. Russell, D. W. et al., Current Topics in Plant Biochemistry, Vol. 4, ed. by D. D. Randall et al., Univ. of Missouri (1985).
The activity of HMG-CoA reductase in animals and yeasts is known to be subject to feedback inhibition by sterols. Such feedback inhibition requires the presence of the membrane binding region of the enzyme. See, e.g., Gil, G. et al., Cell, 41: 249-258(1985); Bard, M. and Downing, J. F. Journal of General Microbiology, 125:415-420(1981).
Given that mevalonate is the precursor for sterols and other isoprenoids, it might be expected that increases in the amount or activity of HMG-CoA reductase would lead to increases in the accumulation of both sterols and other isoprenoids. In yeasts and non-photosynthetic microorganisms, increases in HMG-CoA reductase activity are not associated with predictable increases in the production of sterols or other isoprenoids.
In mutant strains of the yeast Saccharomyces cerevisiae (S. cerevisiae) having abnormally high levels of HMG-CoA reductase activity, the production of two sterols, 4,14-dimethylzymosterol and 14-methylfecosterol, is markedly increased above normal. Downing, J. F. et al., Biochemical and Biophysical Research Communications, 94(3): 974-979(1980).
When HMG-CoA reductase activity was increased by illumination in non-photosynthetic microorganisms, isoprenoid (carotenoid), but not sterol (ergosterol), synthesis was enhanced. Tada, M. and Shiroishi, M. Plant and Cell Physiology, 23(4): 615-621(1982). There are no studies reporting the effects of such increases in HMG-CoA reductase activity in plants.